The use of bias controllers and (e.g. Mach-Zehnder (MZ)) modulators in communications systems is known. Controllers and modulators may be used to modulate an input optical carrier signal with a radio frequency (RF) communications signal.
FIG. 1 is a schematic illustration (not to scale) of an MZ modulator 1.
The MZ modulator 1 provides a mechanism whereby an input optical carrier signal may be modulated with a communications signal, for example with an RF communications signal. In this example the modulator is effectively an interferometer, created by forming an optical waveguide in a suitable substrate such as Lithium Niobate (LiNbO3) or Gallium Arsenide (GaAs) or Indium Phosphide (InP).
A waveguide 11 of the MZ modulator 1 is split into two branches 11a, 11b before recombining at an optical coupler 13. An optical carrier signal in the form of a beam of light enters one side of the modulator 1 (as indicated by an arrow at the left-hand side of FIG. 1) and exits the modulator 1 at the opposite side (i.e. at the right-hand side of FIG. 1) having passed through both branches 11a, 11b of the waveguide 11.
One of the waveguide branches 11a includes an asymmetry 15 that functions to introduce a phase difference between light travelling down respective branches 11a, 11b of the waveguide 11. The phase difference is chosen to be approximately 90 degrees at the wavelength of operation, which is typically in the region of 1300 or 1550 nanometers. This induces quadrature bias where the optical output is nominally 50% of its maximum.
Lithium Niobate (in common with other similar materials such as GaAs or InP) is a glass-like material with a crystal structure that exhibits an electro-optic effect whereby the refractive index of the crystal structure changes as a voltage is applied thereto. In particular, the direction of the electric field induced by the applied voltage causes an increase or decrease in refractive index. An increased refractive index acts so as to slow light travelling through the crystal, and a decreased refractive index acts so as to increase the speed of light travelling through the crystal. In MZ modulators, the Lithium Niobate material is usually arranged so as to have an X cut, Y propagate crystal orientation with respect to the input optical signal, and in this context an electric field applied in the X direction (positive or negative) causes a change in the refractive index of the material that affects the speed of the light passing along the Y axis.
As shown in FIG. 1, a modulating electrode 7 is provided between the branches 11a, 11b of the waveguide 11. When the modulating electrode 7 is energised by an applied signal (e.g. a radio frequency or digital communications signal), positive and negative electric fields are established between the modulating electrode 7 and, respectively, a first 3 and a second 5 ground plane. The modulating electrode 7 is designed as a transmission line so that the modulating signal travels with the optical carrier signal through the MZ modulator 1, thereby enabling high modulating frequencies to be achieved.
The positive and negative electric fields cause the refractive index of the two branches 11a, 11b of the waveguide 11 to change (the positive field causing an increase in refractive index for branch 11a, and the negative field causing a decrease in refractive index for branch 11b), and the resulting different propagation speeds of the optical carrier signal through each branch cause a change in phase in the signals output to the optical combiner 13, which phase change causes the output level of light from the optical combiner 13 to change. In effect, as the electric fields experienced by each branch varies with the communications signal applied to the modulating electrode 7, so the phase difference between light passing through the two branches changes and the output level of the optical signal output from the combiner 13 varies accordingly. The net effect of this is that the input optical carrier signal is modulated with the communications signal applied to the modulating electrode 7.
FIG. 2 is a schematic illustration (not to scale) showing a modulator transfer function. This transfer characteristic of the MZ modulator 1 is approximately sinusoidal. The most linear modulation tends to be achieved in and around the quadrature point (also known simply as “quadrature”). The quadrature point is the point where there is a 90 degree phase relationship between light travelling through respective branches 11a, 11b of the waveguide 11. The transfer function is a repeating function, and as such there are many quadrature points at different bias voltages but all with the same power output. Indicated in FIG. 2 by the reference sign A is a first quadrature point. At this first quadrature point A the output power is increasing with bias voltage, and hence this quadrature point A is referred to as a positive slope quadrature bias point. Indicated in FIG. 2 by the reference signs B and C are two further quadrature points B and C where the output power is decreasing with bias voltage. These quadrature points B, C are each referred to as negative slope quadrature bias points.
In practice, the preferred 90 degree phase shift is rarely, if ever, achieved. To compensate for this, it is usual to include a biasable component 9, and to apply a DC bias voltage to the biasable component 9, to return the MZ modulator 1 to or near to one of the aforementioned quadrature points. In the arrangement depicted in FIG. 1, the biasable component 9 comprises a discrete bias electrode (this is merely illustrative as a number of alternative arrangements are known to persons skilled in the art). For example, a bias voltage may be applied directly to the modulating electrode 7 by means of a so-called bias-Tee. In such an arrangement, the DC bias is coupled to the electrode via an inductor, and the applied signal (for example an RF communications signal) is coupled to the electrode via a capacitor.
A problem with this arrangement is that the bias point, i.e. the voltage that needs to be applied to the biasable component 9 to return the MZ modulator 1 to or near the quadrature point, shifts over time. For example, so-called trapped charges (e.g. that exist in the regions between electrodes, e.g. in a silicon dioxide buffer layer on the surface of the device) and temperature variations can each cause the bias point to shift at a rate of anything from a few millivolts per hour to several volts per hour. Thus, conventionally it tends not to be possible to provide a system where the bias voltage, once set, need not be changed. As such it is usual to provide some sort of dynamic bias control to enable modulator linearity to be maintained over an extended period of time.
In the analogue domain, dynamic bias control has previously been achieved by applying a pilot tone (for example a 10 kHz tone for a multi-GHz communications signal of interest) to the modulating electrode, by monitoring the output of the modulator and by adjusting the bias voltage based on that output. For example, as the 2nd harmonic of the pilot tone usually tends to be minimal at or around the quadrature point, one previously proposed approach monitors this second harmonic and adjusts the applied DC bias voltage to minimise the second harmonic. A similar approach has previously been proposed for the digital domain, but in this instance the signal applied is typically a square wave dither signal, and the output is monitored by a digital signal processor.
Whilst each of these approaches do enable a form of dynamic bias control to be provided, they each have attendant disadvantages. For example, the application of a pilot tone necessarily gives rise to modulation products (for example sidebands) that limit the performance of the system, and for high-fidelity optical links this reduction in performance is unacceptable. In very high-speed links (for example, digital links with speeds of up to 100 GBit/s and analogue links with frequencies of up to 60 GHz), the application of a dither can adversely affect the achievable data rate and the length of link that is achievable. Another disadvantage particularly prevalent in instances where multiple channels are required, for example in a phased array antenna system, is that as each modulator is different the bias control hardware needs to be fully replicated for each and every modulator. This increases system bulk, complexity and cost.
WO 2008/059198 discloses a bias controller for an optical modulator. The modulator includes a bias electrode that is operable when appropriately biased by an applied bias voltage to configure the modulator to operate at quadrature. The bias controller comprises means for generating power signals indicative of the optical output power of the modulator, and a processor operably connected to the generating means and the bias electrode. The processor is arranged to receive the power signals from the generating means and to control the bias voltage applied to the bias electrode. The processor is configured to vary the bias voltage applied to the bias electrode and to determine (from power signals received from the generating means) a peak optical output power for the modulator. The processor is further configured to determine, in dependence upon the peak optical power, a target optical power for quadrature with reference to a store of predetermined values for peak output power and respective corresponding values of target optical power for quadrature.